ISA 562 Assignment 3
Due before class: 07/21/08. No late submissions please.
Please submit electronic copy to: mxu@gmu.edu with subject line “ISA562 HM3”
This is an individual assignment. You are required to solve it on your own using
whatever material you like. Please sign and submit the following honor code statement
with your solution:
I have not taken any help on this examination from anyone and not provided any help to
anyone. The solution has been entirely worked out by me and represents my individual
effort.
Please submit a typed solution with the signed honor code statement.
1. Following the notations introduced in class, translate those messages into English:
(30pt)
(a) AB: {m || r1}kAB || h(m || r1)
(b) AB: {m1}kAB || {m2}kAB || h(m1 || m2)
(c) AB: {m || r1}eB || {m || r1}dA
(d) AB: {m || r1}kAB || {h( m || r1 )}dA
(e) AB: {m || r1 || {h(m || r1)}dA}ks || {ks || r2}eB
2. Consider an SSL handshake (RSA one-way, as discussed in class). (20pt)
(a) Suppose an attacker has intercepted all messages sent by the client during a former
SSL handshake. The attacker now replays the messages to the server (and ignore
whatever the server sends back). Describe when and why the server will detect this attack.
(b) Now the attacker changes his strategy. He/she ONLY replays the messages in the first
three rounds (that is, up to ‘client key exchange’), but he/she repeats this replay for many
times. What is the attacker’s intention? (Hint: consider what the server would do)
3. Suppose Alice and Bob want to establish a shared secret key. They agree on the
following:
n=353 and g=3; Alice chooses Xa=97; Bob chooses Xb=233. Derive the public keys for
both (Ta & Tb) as well as the shared secret key (K). (10pt)
4. Design a Role-based access control system for an organization with the following
requirements. That is, answer the questions following the story (40pt)
The story: Any branch of the GMU bank consists of customers, cashiers, a manager and
auditors.
1. By default, the GMU president is an auditor of the Fairfax branch.
2. A bank manager or a cashier cannot become an auditor.
3. Any person with a valid SSN may become a customer, including all bank
employees (consisting of auditors, cashiers, and a manager)
4. A manager may become a cashier temporarily for a day, but may give up the
manager’s duty for that day, provided that another cashier becomes the manager
for that day. Then, if the permanent manager had approved any transactions
carried out by that cashier (who become manager) cannot approve any transaction
of the permanent manager’s accounts.
5. An auditor may never become a cashier or a manager.
6. A cashier or the manager must relinquish that duty temporarily in order for them
to do their own customer transactions.
7. Alice_1, ……, Alice_27 are customers of the Fairfax branch of the bank. Lloyd
(who holds a joint account with Alice_15) is a manager and Danny and Cindy are
cashiers at the Fairfax bank. The current GMU president is Alain.
8. No employee may approve any activity on an account with his/her name.
9. An auditor may audit any transaction and balance books.
10. A cashier may debit or credit any customer account, provided the customer
submits a written permission.
11. An account may be owned by a single customer or jointly by two of them.
12. A joint account transfer has to be approved by the manager.
Questions:
1. What are the roles of RBAC system that models the bank:
2. What are the permissions in the RBAC system?
3. Who are the subjects of the RBAC system?
4. What is the Role-to-permission map?
5. What is the Subject-to-role-map?
6. What are the static constraints?
7. What are the dynamic constraints?
8. If Lloyd had approved a transaction carried out by Danny for another, and now
wants to do a transaction on the account he holds jointly with Alice_15, who
should become the temporary manager?